Acetylcholinesterase inhibition by Rosmarinic acid from Plectranthus (Lamiaceae) species

Alzheimer disease (AD), a very common form of dementia among the elderly, is correlated with brain deficits in acetylcholine (ACh). Nowadays, the most effective strategy for the symptomatic treatment of AD consists in the inhibition of acetylcholinesterase activity, the enzyme responsible for ACh inactivation and for the increasing of ACh concentration in brain synapses.

Many Plectranthus L'Herit. species are often used in folk medicine to treat a range of diseases, including psychiatric problems. Extracts of P. barbatus Andrews, P. verticillatus (L.f) Druce, P. lanuginosus (Hochst. ex Benth.) Agnew. and P. fruticosus L'Herit. were prepared as decoctions and tested for their ability as acetylcholinesterase inhibitors. HPLC and NMR data showed that rosmarinic acid is the main compound in all the Plectranthus extracts assayed and the acetylcholinesterase inhibitory activity exhibited by the plant extracts is related to its content (Table 1). The HPLC chromatogram of P. barbatus extract showed other compounds than rosmarinic acid, which explains a higher acetylcholinesterase inhibition than the expected for its rosmarinic acid content.

Table 1: Acetylcholinesterase inhibition capacity and rosmarinic acid content in Plectranthus extracts.

Plectranthus species	Acetylcholinesterase Inhibition with 0.5mg/mL of Plant Extract (%)	Rosmarinic Acid Content in 0.5mg of Plant Extract (mg)
P. verticillatus	59.60±7.60	0.222
P. barbatus	31.48±3.25	0.070
P. fruticosus	31.30±2.60	0.144
P. lanuginosus	10.02±1.12	0.098

Acknowledgements: The first author is grateful to FCT (Fundação para a Ciência e Tecnologia) for a doctoral grant (SFRH/BD/37547/2007)